Most substrates or supports of recently developed magnetic recording medium comprise polyethylene terephthalate. Polyethylene terephthalate has a high resistance against organic solvents and excellent mechanical strength because of stretching and high crystallization. The magnetic layer obtained by coating a composition comprising ferromagnetic fine particles dispersed in a binder is mechanically very brittle. Thus such a magnetic layer having an ordinary thickness directly applied onto the substrate without interposing an undercoat or subbing layer therebetween will be readily broken on exerting a slight force. It is very difficult to firmly adhere such magnetic layer to the substrate. Accordingly it is the easiest and the most effective solution that an undercoat or subbing layer is provided between the substrate and the magnetic layer as is done in painting technology.
In order to provide a high density magnetic recording medium having good running properties and an excellent adhesion it is necessary for the magnetic layer to have a low surface electrical resistance and an excellent adhesion without impairing its surface smoothness and the like. With respect to subbing arts, there are known arts as are disclosed in Japanese Patent Publication Nos. 47(1972)-220071 and 49(1974)-10243, Japanese Patent Kokai-Publication Nos. 49(1974)-46406, 49(1974)-46407, 50(1975)-32905, 50-32906 and 50-32907, etc. However these arts have both merits and demerits and do not always meet sufficiently the aforementioned requirements as subbing compositions.